There are over a thousand active satellites in Earth orbit. The missions of these satellites vary greatly in terms of scope and objective, and include missions that provide observation, communication, and positioning services. Traditionally, many satellite programs have focused on a single platform for mission execution. The single platform model is still in widespread use, with current observation missions flown by NASA and NOAA including multi-purpose observing platforms with costs surpassing $3B per satellite. However, modern missions include those executed by constellations of several satellites working in combination. In the field of positioning services, global navigation satellite systems (GNSS) are provided today by constellations of satellites operating in concert such as the GPS, GLONASS, Galileo, and Beidou constellations. Communications missions also traditionally focused on a single platform responsible for broadcasting to a given geographic area. However, that sector has developed to include the use of constellations of satellites, and large constellations of low earth orbit (LEO) satellites using Ku- or Ka-band spectrum are currently being developed to provide diverse satellite-based internet communications. Imaging observation missions are today often conducted by constellations of satellites to increase the area that can be imaged and reduce latency. Furthermore, companies such as GeoOptics are currently developing cubesats for other types of Earth observation missions such as atmospheric radio occultation (RO).
The use of less expensive smaller satellites has proved beneficial from both a cost and performance perspective across a wide range of observation paradigms. For example, in terms of optical imaging improvements, some private companies have utilized cubesats flying in particular formations to provide a more complete and up-to-date image of the Earth, the benefit being that more eyes in more places get more information. However, constellations also provide a benefit in terms of their ability to correlate and compare observations of the same phenomenon to enhance the information return. RO observations have benefited from the use of constellations in this way. RO involves observing the change in a radio signal as it passes through a medium to obtain information regarding that medium. For example, RO conducted with the radio signals produced by GNSS can provide information regarding the Earth's atmosphere. See, T. P. Yunck and G. A. Hajj, Global Navigation Satellite Sounding of the Atmosphere and GNSS Altimetry: Prospects for Geosciences, Jet propulsion Laboratory, CA Inst. Of Tech., Pasadena, Calif., 1987. Current RO missions involving two or more satellites include Germany's TerraSAR-X and Tandem-X radar imaging systems as well as Taiwan's COSMIC system of GNSS RO satellites. In addition to RO, constellations of satellites have also been considered as a potential vehicle for obtaining altimetry and other measurements regarding the Earth's surface. For example, the NASA funded Cyclone Global Navigation Satellite System (CYGNSS) is being designed with the hope of being able to passively observe reflections of GNSS signals off the Earth with a constellation of satellites in order to obtain information regarding ocean surface winds.